MXPA99009341A - Ethylene oxide catalyst - Google Patents

Abstract

A silver catalyst for ethylene oxidation to ethylene oxide is provided containing a promoter combination consisting of an alkali metal component, a sulfur component, and a fluorine component, the catalyst being essentially free of rhenium and transition metal components.

Description

ETHYLENE OXIDE CATALYST Background of the Invention Field of the Invention The present invention relates to a catalyst for the oxidation of ethylene to ethylene oxide consisting of silver, alkali metal such as cesium, fluorine and sulfur deposited on a support such as alpha alumina and the production of ethylene oxide using the catalyst. Description of the Prior Art Processes for the production of ethylene oxide involve the oxidation of the vapor phase of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. Great efforts have been made by many workers to improve the effectiveness and efficiency of the silver catalyst to produce ethylene oxide. The Patent of E.U.A. 5,051,395 provides a comprehensive analysis of the efforts of previous workers. Among many previous teachings in this area is the U.A. 4,007,135 (see also UK 1,491,447) which teaches variable silver catalysts for the production of ethylene and propylene oxides comprised of a promoter amount of copper, gold magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, mo bdene, tungsten, chromium, vanadium and / or preferably barium, in excess of any present immobile in the preformed support as impurities or cements (column 2, lines 1-5), silver catalysts for the production of propylene oxide comprising a promoter amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form of the preformed support as impurities or cements (column 2, lines 16-34), as well as silver catalysts for the production of oxide or propylene oxide comprising (a) an amount promoter of sodium, cesium, rubidium, and / or potassium, and (b) magnesium, strontium, calcium and / or preferably barium in a promoter amount (column 3, lines 5-8). The Patent of E.U.A. 5,057,481, and the related 4,908,343 refer to plant ethylene oxide catalysts which are comprised of cesium and an oxyanion of a group of elements 3b to 7b. ~ The Patent of E.U.A. No. 3,888,889 discloses suitable catalysts for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element of Group 5b and 6b. Although the use of the supports are mentioned, there are no examples. The use of cesium is not mentioned. European Application 0 266 015 deals with supported silver catalysts promoted with rhenium and a long list of possible co-promoters. _ The Patent of E.U.A. 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a support impregnated with silver which also has thereon at least one cation promoter such as cesium and a promoter comprising (i) sulfate anion, (ii) fluorine anion and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table. Possibly for comparison purposes because it is out of the reach of the claimed catalyst, the patent shows in columns 21 and 22 a catalyst No. 6 comprising Ag / Cs / S / F on a support, the amount of Cs being 1096 ppm. In the context of the large and a vast number of references, many of them contradictory, the applicant has discovered a novel and improved catalyst for the production of ethylene oxide. * Brief Description of the Invention The present invention relates to an improved, supported silver ethylene oxide catalyst containing a promoter combination consisting of a critical amount of an alkali metal component, preferably cesium, together with a component of sulfur and a fluorine component and to the preparation and use of catalyst; the catalyst being essentially free of rhenium and transition metal components. Detailed Description-The preferred catalysts prepared according to the invention contain up to about 3% by weight of silver, expressed as metal, deposited on the surface and through pores of a porous refractory support. The silver content of less than 20% by weight of the total catalyst is effective, but results in catalysts that are unnecessarily expensive. The content of silver, expressed as metal, is about 5-20% based on the weight of the total catalyst, while silver content of 8-15% is especially preferred. In addition to silver, the catalyst of the invention also contains a critical promoter combination consisting of certain amounts of alkali metal, sulfur and fluorine. The critical amount of the alkali metal promoter component is not greater than 800 ppm expressed as the alkali metal based on the weight of the catalyst; preferably the catalyst contains 400-800 ppm, more preferably 500-700 ppm alkali metal based on the weight of catalyst. Preferably, the alkali metal is cesium through lithium, sodium, potum, rubidium and mixtures can also be used. The impregnation processes are described in US Pat. No. 3,962,136, advantageously they are used for the addition of the cesium component to the catalyst. Also necessary for the practice of the invention is the provision of sulfur as a component of promoter catalyst. The sulfur component can be added to the impregnation solution of impregnation support such as sulfate, e.g., cesium sulfate, ammonium sulfate and the like. The patent of E UA 4 766 105 describes the use of sulfur promoting agents, for example in column 10, lines 53-60, and this description of incofora here by reference. The use of sulfur (expressed as the element) in the amount of 5-30,000 ppm by weight based on the weight of the catalyst is essential according to the invention. The catalyst also contains a fluorine promoter in an amount expressed as element F of 10-300 ppm by weight based on the catalyst as an essential component. Ammonium fluoride, alkali metal fluoride and the like can be used. The catalysts are formed with supports comprising alumina, silica, silica-alumina or combinations thereof Preferred supports are those which mainly contain alpha-alumina, particularly those containing up to about 15% by weight of silica. Especially preferred supports have a porosity of about 0.1-1 10 cc / g and preferably about 0.2-0.7 cc / g.The preferred supports also have a relatively low surface area, ie of about 0.2-2.0 m 2 / g, preferably 0.4-1.6 m2 / g and more preferably 0.5-1.3 m2 / g as determined by the BET method See J. Am. Chem. Soc. 60, 3098-16 (1938) Porosities are determined by the mercury porosimeter method, see Drake and Ritter, "Ind. Eng. Chem. Anal. Ed. "17, 787 (1945) .Pore diameter and pore distributions determine surface area and apparent porosity measurements.

For use in commercial ethylene oxide production applications, supports are conveniently formed in the formation of regulating pellets, spheres, rings, etc. Conveniently, the support particles can have "equivalent diameters" in the range of 3-10 mm and preferably in the range of 4-8 mm, which are usually compatible with the internal diameter of the tubes in which the catalyst is placed. The "equivalent diameter" is the diameter of a sphere that has the same external surface (ie, the negligible surface within the pores of the particle) to the volume ratio as the support particles being used. Preferably, the silver is added to the support by immersing the support in a silver / amine impregnation solution or by the incipient wetting technique. Silver containing liquids penetrated by absorption, capillary action and / or vacuum in the pores of the support. A single impregnation or series of impregnations, with or without intermediate drying, can be used, depending in part on the concentration of the silver salt in the solution. The obtained catalyst has silver contents within the preferred scale, the impregnation solutions generally contain 5-50% by weight of silver, expressed as metals. The exact concentrations employed, of course, depend, among other factors, on the desired silver content, the nature of the support, the viscosity of the liquid and the solubility of the silver compound.

- The impregnation of the selected support is achieved in a conventional way. The support material is placed in complete silver solution of the solution is absorbed by the support. Preferably the amount of the silver solution used to impregnate the porous support no more than that necessary to fill the pore volume of the porous support. The impregnation solution, as already indicated, is characterized as a silver / amine solution, preferably as described in the U.S. Patent. 3,702,259 the description of which is incorporated herein by reference. The impregnation processes are described in Ta Patent of E.U.A. 3,962,136 advantageously employed of the cesium component. Several of the known prior methods of predisposition, co-arrangement and post-disposal of several of the promoters can be employed. . After impregnation, any excess impregnation solution is separated and the silver impregnated support and the promoter or promoters are calcined or activated. In the most preferred practice of the invention, the calcination is carried out as in the patent of E.U.A. 5,504,052 commonly assigned filed on April 2, 1996 and copending application Series No. 08 / 587,281 filed on January 16, 1996, the descriptions of which are incorporated herein by reference. The calcination is achieved by heating the impregnated support, preferably at a gradual rate, at a temperature in the range of 200-500 ° C for a sufficient time to convert the silver contained in the silver metal and to decompose the organic materials and remove the same as volatiles. The impregnated support is maintained under an inert atmosphere while it is above 300 ° C throughout the procedure. While it is not desired to be bound by any theory, it is thought that at temperatures of 300 ° C and higher oxygen, it is absorbed in substantial amounts in the volume of the silver where it has an adverse effect on the characteristics of the catalyst. The inert atmospheres employed in the invention are those that are essentially free of oxygen. - An alternative method for calcination is to heat the catalyst in a stream of air at a temperature not exceeding 300 ° C, preferably not exceeding 250 ° C. The catalysts prepared according to the invention have improved performance, especially stability, for the production of ethylene oxide by the oxidation of the vapor phase of ethylene with molecular oxygen. This usually involves reaction temperatures of approximately 150 ° C to 400 ° C, usually from about 200 ° C to 300 ° C, and the reaction pressures in the range from 0.5 to 35 bar. Reagent feed mixtures contain from 0.5 to 20% ethylene and from 3 to 15% oxygen, with the equilibrium comprising comparatively inert materials including substances such as nitrogen, carbon dioxide, methane, ethane, argon and the like.

Only a portion of ethylene is usually reacted per pass on the catalyst and after removal of the desired ethylene oxide product and removal of the appropriate purge streams and carbon dioxide to prevent uncontrolled accumulation of inert materials and / or unreacted byproducts are returned to the oxidation reactor. The following examples illustrate the examples of the invention. Example 1"A silver solution was prepared using the following components (parts by weight): Silver oxide - 834 parts Oxalic acid - 442 parts Deionized water - 2808 parts Ethylenediamine - 415 parts Silver oxide was mixed with water, at room temperature The mixture was stirred for 15 minutes and at this point the color of the black suspension of the silver oxide changed to the gray / brown color of silver oxalate. solids were washed with 3 liters of deionized water. = The vessel containing the washed solids was placed in an ice bath and stirred while the ethylene diamine and the water (as a mixture of 72% / 28%) were added slowly in order to keep the reaction temperature lower than 33 ° C. After the addition of the entire water / ethylenediamine mixture, the solution was filtered at room temperature. The clear filtrate was used as a silver / amine stock solution for catalyst preparation. Support was obtained used for the Norton Company examples and was mainly made from ajfa-alumina in the form of cylinders of 0.79 cm. The support has a surface area of 0.65 m2 / g, pore volume of 0.3 cc / g and average pore diameter of 1.5. For Example 1, about 185 parts of the silver solution were mixed with: 1. 1.89 pairs of CsOH solution (8% Cs by weight in water), 2. 1.03 parts of amino fluoride, (3% F) in weight in water) and 3. 1.6 parts of ammonium acid sulfate, (1% of S by weight in water). The mixture was stirred to ensure homogeneity, then added to 400 parts of the support. The wet catalyst was mixed for ten minutes and then calcined. Examples 2-16 (Table 1) were prepared following the same procedure described in Example 1, except that the amounts of the solutions of Cs, S and Normal F's were adjusted in order to obtain the desired levels of promoters. The calcination of the deposit of the silver compound was induced by heating the catalyst to the decomposition temperature of the silver salt. This was achieved via ^ heating in an oven that has severe heating zones in a controlled atmosphere. The catalyst was loaded onto a moving band that entered the furnace at room temperature. The temperature increased gradually as the catalyst moved from one zone to the next. If it increased, up to 400 ° C, as the catalyst passes through seven heating zones. After the heating zones the band passed through a cooling zone which gradually cooled the catalyst to a temperature below 100 ° C. The total residence time was controlled through the use of nitrogen flow in the different heating zones. In some cases, as indicated in the following table, the calcination was carried out with air. The catalyst was tested in a tube which was heated by a salt bath. A gaseous mixture containing 15% ethylene, 7% Oxygen and 78% nitrogen and inert carbon dioxide, was passed through the catalyst at 21.09 kg / cm2. The temperature of the reaction was adjusted in order to obtain the productivity of ethylene oxide of 160 kg. per hour per m3 of catalyst and its temperature is given in the Table. '- - "The results of the tested catalyst are summarized in Table 1.

Table 1 NO * Comparative t CJ1 O n in Table 1 (continued) ? 'Comparative In Comparative Example 2, the case of normal Ag / Cs, the selectivity of the catalyst is 81.5%. This performance did not improve the addition of F or S alone or the addition of these promoters in amounts lower than the optimum level, comparative examples 1, 3 and 49. In addition to all the promoters at the optimum level gave the superior performance, examples 5 and 9. The inferior results are achieved if F and / or S exceed the optimal levels, examples 13-16. The high selectivity of Example 9 can not be correlated only with the high level of Cs because the concentration of Cs was increased to the level of 600 ppm. Without the presence of the optimum level of both S and F, the catalyst had poor selectivity and activity, see example 6.. Actually, the presence of S and F have a profound effect on the optimal level of Cs. In the case of the Ag / Cs catalyst, the optimum level of Cs is 300 ppm. The lower Cs level leads to lower activity and selectivity, compare examples 2 and 6. In addition to the S and F arrows for the addition of high Cs level without loss of performance; compare examples 7, 8 and 10. The data shows that S and F increase the selectivity for the 83.5% scale, see examples 5, 9 and 12. It is clear that 600 ppm is the new optimum concentration and Cs higher than 770 ppm, which will lead to decreased performance, examples 11, 14 and 16. The data further shows that the preferred method of calcination is to heat the catalyst in an inert atmosphere. see examples 9 and 10

Claims (6)

1. CLAIMS? A catalyst for the oxidation of ethylene to ethylene oxide comprised of silver on a solid support and containing a promoter combination consisting essentially of (1) an alkali metal component in an amount not greater than 800 ppm, (2) a component of sulfur in an amount of 5-300 ppm and (3) a fluorine component in an amount of 10-300 ppm.
2. 2. The catalyst of claim 1, wherein the alkali metal component is cesium.
3. 3. The catalyst of claim 2, wherein the cesium component is an amount of 400-800 ppm.
4. 4. The catalyst of claim 1, wherein the support is alpha alumina.
5. 5. The catalyst of claim 1, comprised of 5-20% by weight of silver.
6. 6. The method for producing ethylene oxide comprising reacting ethylene and molecular oxygen in the presence of the catalyst of claim 1.